1. Field of the Invention
In electronic article surveillance (EAS) systems, articles being protected are tagged with a tag containing an electronically detectable device which is referred to as a marker. Typically, a sweep frequency interrogation transmitter whose frequency is swept through a resident frequency of the tag includes a transmitting antenna located near an exit of a protected area. A receiving antenna is located near the transmitting antenna and forms a passage-way with the transmitting antenna through which someone exiting the protecting area must pass. The receiving antenna is coupled to a receiver that detects the signal radiated by the marker whenever the transmitter frequency passes through the resident frequency of the marker. There are two types of EAS systems of primary use commercially. Radio frequency (RF) systems and magnet systems. The instant invention has particular use in a magnetic EAS system wherein a magnetic field is generated at a fixed frequency. A signal is generated when the magnetic field causes the magnetization of a marker to switch. This occurs near zero field amplitude.
2. Description of the Related Art
Although there are many EAS systems that work satisfactorily well, all these systems face the problem of distinguishing a signal emitted from the EAS marker from background noise. For example, in U.S. Pat. No. 5,023,598, detection is achieved by the use of averaging techniques of a plurality of sweeps wherein peaks above a defined level are stored in a persistent table. A symmetry test is made on the peaks and if the peaks are persistent and symmetrical, the presence of a marker is indicated. Although this system works well, it is primarily directed to radio frequency systems that detect the presence of a resonant tank circuit. U.S. Pat. No. 5,005,001 describes an EAS system that has a signal generator for generating a magnetic field which includes an arrangement for generating a non-rotating field at a first frequency and a rotating field at a second frequency that is lower than the the first frequency. This system is designed for the purpose of detecting magnetic markers and represents an advancement in the constant attempt to eliminate background noise; nevertheless, it would be advantageous to reach a higher level of efficiency for detecting magnetic markers.